This invention relates to positioning systems, and it relates more particularly to such systems as can be used to supplement positional information derived by means of a Global Positioning System (GPS), so as to provide positional data coverage in situations where the GPS data may be unreliable, for example due to environmental attenuation of satellite signals.
The invention finds especial, though not exclusive, application in circumstances where several members of a group of people, each having their own GPS, enter together terrain in which for some reason, for example as a result of its being thickly wooded or otherwise highly vegetated, heavily attenuates the satellite signals, receipt of which is necessary for the OPS to function. In the event of a loss of the GPS data, the members of the group can lose touch both with one another and, in certain situations, this can be hazardous, since one or more of the members may become disorientated and lost and they may stray into a dangerous or hostile environment. Moreover, should any of the people in the group become ill or injured, assistance may be delayed in reaching them because their whereabouts are not known, or are not known to sufficient accuracy.
It is feasible for each person in the group to track and store the absolute positional information, derived from GPS data, relating to all of the other members and, in the event of failure of the communication channels on which GPS relies, to extrapolate from the last known positions using data derived from an inertial navigation system, provided that all members of the group are equipped with such systems and the wherewithal to use the results so provided.
However, since inertial navigational systems are based upon measuring accelerations detected by an accelerometer, and such detected information has to be integrated twice in order to produce positional data, any measurement error associated with the accelerometer becomes multiplied by the square of the time. Thus, position errors accumulate exponentially with time.
Of course, the use of highly accurate accelerometers can mitigate this problem, to some extent at least, but such accelerometers tend to be bulky and expensive, thus impractical for use in the field by individual group members.
This invention seeks to address the above problem and, according to the invention from one aspect, there is provided a positioning system for indicating to each member of a group information indicative of the respective locations of other members of the group, wherein each said member is provided with relatively inexpensive and readily transportable inertial navigation equipment in order to provide positional information, active means to generate additional positional data, and means utilising said additional data to compensate, at least in part, for inaccuracies in the positional information provided by the inertial navigation equipment.
In one preferred embodiment, the inertial guidance equipment is based upon solid state micro electro-mechanical systems (MEMS) accelerometers, as such devices are light, compact and relatively inexpensive.
Preferably, the active means to generate said additional positional data includes a two-way radio system; the signals transmitted by all of the radio systems being distinguishable from one another. By this means, with relatively lightweight and inexpensive equipment, two or three-dimensional corrective data can be applied to the positional information as to the locations of the respective group members, provided by their inertial navigation systems.
In preferred embodiments, the radio systems and/or processing means associated therewith include means for determining the distance travelled by radio signals received from other group members.
The distance determination based upon the reception of radio signals may conveniently be based upon phase measurements of signals locked in phase to that of a carrier originally transmitted by a receiving radio. Alternatively, the distance measurement may be derived by correlating transmissions and receptions with an accurately synchronised time base or, as a further alternative, pulsed transmissions can be used.
It is further preferred that the radio signals are used to compute a mean position for the members of the group, against which the inertial navigational data can be related to effect said compensation.
In any event it is preferred that the group members are all provided with individual GPS equipments and the system of the invention is used to provide information as to the whereabouts of the group members during periods when GPS is inoperative.
The system may be actuated manually in response to observance of GPS failure. Preferably, however, the GPS failure is detected automatically and the system is energised automatically.
In some circumstances it is preferred that the system be energised periodically, even when the GPS system is functioning, in order to provide for a self-check and to monitor its performance against the GPS. In instances where inadequate performance is detected, an audible and/or visual warning is preferably provided to alert the user to the fact that the back-up system is under-performing, and that the system should be checked.
According to another aspect of the invention there is provided a method for a group of mobile objects to determine the absolute position of each member of the group, the method comprising: using an inertial navigation system to generate data indicative of the absolute positions of the objects; receiving signals transmitted between the mobile objects and processing the signals to generate data indicative of the relative positions of the objects; processing the relative position data to generate corrective data; and correcting the absolute position data in accordance with the corrective data.
According to another aspect of the invention there is provided an apparatus for use by a group of mobile objects to determine the position of each member of the group, the apparatus comprising: an inertial navigation system for generating data indicative of the absolute positions of the objects; equipment for receiving signals transmitted between the mobile objects and processing the signals to generate data indicative of the relative positions of the objects; and processing means for processing the relative position data to generate corrective data and correcting the absolute position data in accordance with the corrective data.